Cell Over Charging in JK BMS

[karthik_bkv]

Hello All,

When charging the 8s 100Ah lithium-ion battery with a JK BMS with an active balancer, it's noticed that the cell voltage difference is too high, leading to 2-3 cells being overcharged and triggering a cut-off in the BMS. Is this expected behavior, and are there any solutions to fix it?

I'm completely new to Lithium ion, used lead acid in the past

Attached the cell voltage difference as a reference

 

[AshleyL]

..........Err....did you top balance those 8 cells before putting it in series?

If not....you gonna need to keep the voltage constant at 27V and activate the balancer early at 3.4V.
Once the deviation starts to drop to less than 20mV, increase the voltage slowly to 27.2V, repeat, 27.4v, finally move to 27.6V.

I got a feeling your JKBMS is 1A active balancer model.....with 1A balance current....it gonna take quite a time to balance them all.

 

[karthik_bkv]

..........Err....did you top balance those 8 cells before putting it in series?

If not....you gonna need to keep the voltage constant at 27V and activate the balancer early at 3.4V.
Once the deviation starts to drop to less than 20mV, increase the voltage slowly to 27.2V, repeat, 27.4v, finally move to 27.6V.

I got a feeling your JKBMS is 1A active balancer model.....with 1A balance current....it gonna take quite a time to balance them all.

Hi Ashley,
Thanks for your response

Yes the cells were top balance before putting them,

BMS is 0.6A balancer

 

[AshleyL]

Hi Ashley,
Thanks for your response

Yes the cells were top balance before putting them,

BMS is 0.6A balancer

That weird.
So, what is your charging voltage?
One does not simply charge at 29.2V for 8 cells lifepo4.
3.45v per cell is sufficient, aka 27.6V.

 

[karthik_bkv]

That weird.
So, what is your charging voltage?
One does not simply charge at 29.2V for 8 cells lifepo4.
3.45v per cell is sufficient, aka 27.6V.

Charging voltage set in MPPT charge controller 27.6v

 

[RCinFLA]

Assuming you have overvoltage set properly and you are talking about a cell exceeding the overvoltage trigger voltage, there is a known issue that will allow this condition to happen.

It has to do with MOSFET charge path shut down when overvoltage on a cell is exceeded.

The MOSFET arrangement to make bi-directional blocking is back to back series connected MOSFET's. One blocks the charge path (overvoltage cell triggered), one blocks the discharge path (over discharge current triggered).

When one MOSFET in the series back to back arrangement is disabled, there is a diode voltage drop in the unblocked current flow direction.

The extra diode voltage drop can cause excessive heating on MOSFET's for discharge current if discharge current is too great.

To solve this possible overheating issue, the BMS measures discharge current via the current shunts and microcontroller ADC. The current measurement also has current direction polarity indication. If discharge current exceeds 3 to 5 amps the BMS will override the charge blocking on the shutdown, turned off MOSFET's to eliminate diode voltage drop and prevent too much heating.

The BMS then relies on current magnitude and direction to detect when discharge current is dropping and heading for charging direction again, so it can re-disable charge MOSFET, again preventing any further charging.

Issue is the detection and control of the MOSFET is relatively slow. If discharge current hovers near and below 3-5 amp level, the charge blocking override chatters ON and OFF. Because of the slow turn off time to charge blocking MOSFET this can result in a short time period of charge current squirt into battery.

At a low level of discharge current and when potential charge current greatly exceeds discharge current, this chattering bypass of charge blocking, can repeatedly push short squirts of charge current to battery causing an over-voltaged cell to continue to rise in level.

There is a Youtube video showing this happening. Search for Youtube video titled " Jk BMS failed to protect battery ".

 

[AshleyL]

Charging voltage set in MPPT charge controller 27.6v

......What is your balance start voltage?
That pattern kinda similar to cell inbalance pattern after turning on the balancing 24/7 non stop.

The balance voltage should be kept at 3.45v normally.

 

[karthik_bkv]

......What is your balance start voltage?
That pattern kinda similar to cell inbalance pattern after turning on the balancing 24/7 non stop.

The balance voltage should be kept at 3.45v normally.

Cell imbalance only on full charge, not on discharge or 80% charge

 

[RCinFLA]

Dont't set start of balancing below 3.40v.

Doing so increases the chance of balancer actually misbalancing cells due to discharge current overpotential voltage slump causing voltage sampling of cells to report cell voltages that do not represent which cell has greater SoC. The cell's differences in overpotential voltage slump due to discharge current is greater and dominates over SoC cell voltage differences.

Setting start of balance at 3.40v or greater pretty much guaranties balancing only when there is charge current flowing since getting above 3.4v cell voltage pretty much requires cells see charging current. The voltage delta between cells vs SoC accentuates much more when a cell is above 3.4v making it much easier for BMS to distinguish which cells have greater SoC.

 

[karthik_bkv]

Dont't set start of balancing below 3.40v.

Doing so increases the chance of balancer actually misbalancing cells due to discharge current overpotential voltage slump causing voltage sampling of cells to report cell voltages that do not represent which cell has greater SoC. The cell's differences in overpotential voltage slump due to discharge current is greater and dominates over SoC cell voltage differences.

Setting start of balance at 3.40v or greater pretty much guaranties balancing only when there is charge current flowing since getting above 3.4v cell voltage pretty much requires cells see charging current. The voltage delta between cells vs SoC accentuates much more when a cell is above 3.4v making it much easier for BMS to distinguish which cells have greater SoC.

Thanks for letting me know, I will make the changes as suggested.

 

[karthik_bkv]

View attachment 204848
Cell imbalance only on full charge, not on discharge or 80% charge

Hello All,

When charging the 8s 100Ah lithium-ion battery with a JK BMS with an active balancer, it's noticed that the cell voltage difference is too high, leading to 2-3 cells being overcharged and triggering a cut-off in the BMS. Is this expected behavior, and are there any solutions to fix it?

I'm completely new to Lithium ion, used lead acid in the past

Attached the cell voltage difference as a reference

Today's value, the cell voltage difference is too high at this point, before that the voltage difference is 0.05V.

Can anyone help me on this?

 

[gotbeans]

Today's value, the cell voltage difference is too high at this point, before that the voltage difference is 0.05V.

Can anyone help me on this?

RC did help in the post above, here's the youtube video he was talking about:

If this is the case with your BMS I would just set your charge controller to not go over 27.6 volts or something. Your cell 8 seems to have way lower internal resistance or maybe lower capacity of the other cells and thus it is taking charge faster.
I'd expect the more it over volts the worse this gets?

Is your BMS going on/off like the video is?

 

[karthik_bkv]

RC did help in the post above, here's the youtube video he was talking about:

If this is the case with your BMS I would just set your charge controller to not go over 27.6 volts or something. Your cell 8 seems to have way lower internal resistance or maybe lower capacity of the other cells and thus it is taking charge faster.
I'd expect the more it over volts the worse this gets?

Is your BMS going on/off like the video is?

Yes, happened exactly same as shown in the video!

Cell 8 goes upto 3.700v, all the cells in the battery bank is new

Is this a BMS failure?

 

[gotbeans]

Yes, happened exactly same as shown in the video!

Cell 8 goes upto 3.700v, all the cells in the battery bank is new

Is this a BMS failure?

Not a BMS "failure" but definitely a bug in design

Not sure what to do for you, the inverter can be set to 27.6 and maybe set your cell OVP to like 3.45 so it slows down the charging past that point and it gives you time to balance the cells. And set the start balance at like 3.42 cuz your one cell is going to hit that before the others do so let it start draining at that point?
Kinda bad fix there, not sure there is one if there's no actual timer setting.
Once it hits that over voltage if you could make it stop for like 5-10 minutes that may be better.

The settings "charge OCP delay" and "Charge OCPR time"
@RCinFLA Do you know is there a setting like these for the over voltage so you can set a 5min timer for it to not flip back on or something?

 

[AshleyL]

Today's value, the cell voltage difference is too high at this point, before that the voltage difference is 0.05V.

Can anyone help me on this?

Why would you set the charging voltage so high? Stick with 27.2V, slowly increases it to 27.4v and finally 27.6v.

 

[karthik_bkv]

Not a BMS "failure" but definitely a bug in design

Not sure what to do for you, the inverter can be set to 27.6 and maybe set your cell OVP to like 3.45 so it slows down the charging past that point and it gives you time to balance the cells. And set the start balance at like 3.42 cuz your one cell is going to hit that before the others do so let it start draining at that point?
Kinda bad fix there, not sure there is one if there's no actual timer setting.
Once it hits that over voltage if you could make it stop for like 5-10 minutes that may be better.

The settings "charge OCP delay" and "Charge OCPR time"
@RCinFLA Do you know is there a setting like these for the over voltage so you can set a 5min timer for it to not flip back on or something?

This is my current settings

 

[karthik_bkv]

Why would you set the charging voltage so high? Stick with 27.2V, slowly increases it to 27.4v and finally 27.6v.

I've changed the Charging voltage to 27.4v let see if it behaves similar tomorrow.

Thanks for your input.

 

[karthik_bkv]

Current status, with no Solar Charge, seems the cell are balanced, the Voltage difference between the cells are reducing gradually.

 

[karthik_bkv]

Changing the Cell will help to fix this?

 

[AshleyL]

Current status, with no Solar Charge, seems the cell are balanced, the Voltage difference between the cells are reducing gradually.View attachment 205046

......I am starting to question if you know what is top balancing......